The main objective of power system planning is maintaining an adequate level of reliability. The reliability concept has two characteristics: resource adequacy and operational reliability. Traditionally these two characteristics are addressed separately through a sequence of probabilistic and deterministic analyses without a systematic view. This paper presents a resource planning problem that explicitly models the system reliability requirement as chance constraints. Using this formulation, we examine the existing resource planning process to reveal its underlying assumptions. Solution methodology for the problem is also explored. The proposed formulation provides an analytic foundation for designing new planning procedures in a competitive environment and paves the way to a fully probabilistic planning paradigm.